Metal cutting apparatus and method for damping feed-back vibrations generated thereby

ABSTRACT

Feed back vibrations of a metallic tool generated during the machining of metallic workpieces are damped by detecting an oscillatory motion of the tool, identifying a frequency of the oscillatory motion and generating a mechanical camping force having the same frequency as the oscillatory motion and applied to the tool in counter-direction to a velocity of the oscillatory motion. The damping force can be of constant amplitude or gradually decreasing amplitude.

[0001] This application claims priority under 35 U.S.C. §§ 119 and/or365 to patent application Ser. No. 0004540-1, filed in Sweden on Dec. 8,2000, the entire content of which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates to the damping of feed-backvibrations generated by a metal cutting tool during the chip-removingmachining of metal. In particular, the invention relates to a method ofsuch damping which involves:

[0003] detecting the tool's oscillatory motion by means of a sensordevice associated with a control device,

[0004] identifying the oscillatory motion's frequency, amplitude andphase by means of the control device, and

[0005] by means of the control device, actuating a vibration dampingdevice, so that the same renders a mechanical damping force withsubstantially the same frequency as the tool's oscillatory motion.

[0006] The invention also relates to a vibration damper and a mechanicalapparatus for performing the method.

BACKGROUND

[0007] Metallic tools for chip-forming machining can be exposed tovibrations induced by either forces or regenerative oscillations (i.e.,feed-back vibrations); see “Metal Cutting Theory and Practice” byStephenson and Agapiou; publisher Marcel Dekker Inc.; ISBN:0-8247-9579-2. According to this document, vibrations induced by forcesare generated by transient cutting forces, whereas regenerativevibrations (also called chatter or oscillations) occur because thedynamic cutting process forms a closed loop. The present inventionrelates to the damping of only regenerative oscillations (also calledself-induced vibrations or feedback vibrations).

[0008] Regenerative vibration is so named because of the closed-loopnature of the dynamic cutting process. Each tool pass leavesdisturbances in the form of undulations on the workpiece due to thevibrations of the tool and workpiece, and those disturbances producemechanical feedback vibrations in subsequent passes of the tool. Thus,regenerative vibration can be described as a small transient wave thatis machined into the workpiece. That small wave will become the drivingforce, causing the system to vibrate increasingly again in subsequentpasses, i.e., the vibrations from one pass are amplified by those of thenext pass. Unstable conditions can cause a small wavelet to develop andincrease around the circumference, giving non-acceptable machiningresults.

[0009] Damping of vibration in tools for chip removing machining haspreviously been achieved by pure mechanical damping, i.e., the toolshaft being formed with a cavity in which is disposed acounter-oscillating mass of, for instance, heavy metal. The weight andlocation of the mass is tuned in order to provide damping ofoscillations within a certain range of frequencies. The cavity is thenfilled with a viscous liquid, e.g. oil, and is plugged. However, thistechnique works passably only in those cases where the overhang of theshaft from a fastening device is approx. 4-10 times longer than thediameter thereof. In addition to this limitation, the pure mechanicaldamping has an obvious disadvantage in that the range of frequencieswithin which the damping acts, is very limited. An additionalinconvenience consists of the strength-wise weakening of the shaftresulting from the presence of that cavity.

[0010] In entirely other areas of technology, the development of moreefficient, adaptive damping techniques based on the utilization of,among other things, piezo elements has been started. A piezo elementconsists of a material, most often of a ceramic type, which whencompressed or elongated in a certain direction (the direction ofpolarization), generates an electric field in the same direction. Thepiezo element usually has the shape of a rectangular plate having adirection of polarization, which is parallel to the major axis of theplate. By connecting the piezo element to an electric circuit, includinga control module, and compressing or elongating the piezo element in thedirection of polarization, an electric current will be generated andflow in the circuit. Electric resistive components included in thecontrol module will generate heat according to known physics. In doingso, vibration energy is converted to thermal energy, whereby a passivelydamping, but not damping effect on the vibrations is obtained. Moreover,by forming the control module with a suitable combination of resistiveand reactive components, so-called shunts, selected frequencies can bebrought to be damped with particular efficiency. Advantageously, suchfrequencies are the so-called “own-frequencies” of the exposed“own-modes” of the object, which are those preferably being excited.

[0011] Conversely, a piezo element may be compressed or elongated by anelectric voltage being applied over the piezo element, during which thesame may be used as a control or operating device (actuator). This canbe used for active vibration control by selecting the polarity of theapplied electric voltage in such a way that the mechanical stress of theoperating device acts in the opposite direction, as an external,mechanical stress. The emergence of vibrations is suppressed by the factthat another kinetic energy, for instance rotation energy, is preventedfrom being converted to vibration energy. In doing so, thesynchronization of the applied electric voltage relative to the externalmechanical stress, the effect of which is to be counteracted, takesplace by supplying a feedback signal from a deformation sensitive sensorto a control means in the form of a logical control circuit, e.g. aprogrammable micro-processor. The processor processes the signal tocontrol the electric voltage applied over the operating device. Thecontrol function, i.e. the relation between the input signal from thesensor and the output voltage, may, in that connection, be made verycomplex. For instance, a self-learning system for adaptation to varyingconditions is feasible. The sensor may consist of a separate,deformation sensitive device, e.g. a second piezo element, or be commonwith the operating device.

[0012] Examples of practical applications and present development areasfor the utilization of piezo elements for vibration damping purposes,are described in Mechanical Engineering, Nov. 1995, p. 76-81. Thus, skisfor alpine skiing (K2 Four ski, K2 Corp., USA) have been equipped withpiezo elements for the purpose of suppressing undesired vibrations,which otherwise decrease the contact with the ground, and thereby reducethe skier's prospects of a stable and controlled skiing. Furthermore,applications such as increased wing stability of aeroplanes, improvedcomfort in motor vehicles, suppression of vibrations in helicopter rotorblades and shafts, vibration control of process platforms for flexiblemanufacture, and increased accuracy of military weapons are mentioned.In information documents from Active Control experts (ACX) Inc., USA (amanufacturer of piezo elements), the vibration control of snowboards isalso mentioned.

[0013] A method of the kind described in the introduction, as well assuch a vibration damper and such a mechanical structure, respectively,is known from SE-A-9900441-8 (corresponding to U.S. application Ser. No.09/913,271, the disclosure of which is hereby incorporated by referenceherein).

[0014] This type of vibration damper is not suitable for force-inducedvibrations, but only for regenerative, i.e. feed-back vibrations, which,e.g., arise in a tool during mechanical machining when a smalldisturbance gives a mechanical feed-back in the tool. Such a mechanicalfeed-back may cause an increasing oscillatory motion, and thereby anundesired uneven surface of the machined blank and reduced service lifeof the tool.

[0015] While SE-A-9900441-8 does not explicitly describe how to applythe damping force to the mechanical structure, the hitherto known way todampen an oscillatory motion has been to generate a counter force inphase with the oscillatory motion. This procedure works well as long aslow frequencies are concerned. At higher frequencies, i.e. aboveapproximately 500 Hz, it is difficult to apply a counter force withoutphase errors. If a phase error arises, there is a risk of theoscillatory motion and the damping force ending up in an unbalancedstate, and thereby partly amplifying each other, which in turn may leadto the oscillatory motion not being quenched to the desired degree.Thus, a presumption for such a vibration damping to work is that thecounter-directed force is in phase with the oscillatory motion with ahigh degree of accuracy.

[0016] Other piezoelectric dampers are described in SE-A-9803605-6,SE-A98003606-4,SE-A-9803607-2, US-A-4 849 668, DE-A-199 25 193, EP-A-0196 502, US-A-5 485 053 and JP-A-63180401.

[0017] During chip removing machining, such as turning or drilling, itis not unrare for problems with vibrations to arise, particularly incases in which the length of the shaft or tool outside the fasteningdevice (a so-called overhang) is at least 3 times larger than thediameter thereof. One type of vibration is bending vibration, the shaftbeing curved to and fro and submitted to bending deformations. Thisphenomenon constitutes a common problem, for instance during turning,especially internal turning, where the shaft in the form of a boring barhas to be long in order to reach the area in the workpiece which is tobe machined, at the same time as the diameter of the bar is limited bythe dimension of the bore in which machining is carried out. During suchdrilling, turning and milling operations, where the distance to theworkpiece is large, extenders are used, which frequently causes bendingvibrations which not only lead to impaired dimensional accuracy andirregularities in the workpiece, but also to reduced service life of thetool and the cutting inserts or machining elements thereof.

SUMMARY OF THE INVENTION

[0018] The purpose of the present invention is to improve the control ofa vibration-damping device for cutting tools having a regenerativeoscillatory motion.

[0019] This has been attained by a method and a vibration damper,respectively, according to the kind described in the introduction, thecontrol device of which being arranged to generate a damping forcedirected opposite to the cutting tool's oscillatory velocity. This meansthat the damping force gives the tool a deformation directed in theopposite direction to that of the tool's oscillatory motional velocity.

[0020] In this way, a damping of the regenerative motion is obtainedwithout risk that the oscillations end up in imbalance, and therebyamplify the oscillatory motion. The invention does not require a highdegree of accuracy as regards the phase relationship between thefrequencies of the vibration and the imposed damping force, i.e. thedamping force should constantly resist the velocity and in order toobtain a maximum damping effect, a maximum damping force is imposed allthe time. However, it is of minor importance with a maximum force at theoscillatory motion's end points, since the velocity there is low. Thus,it is important that the damping effect is greater than the contributionfrom the cutting process so that the regenerative oscillation is dampedout and a smooth cutting process is obtained by means of the mechanicalstructure, e.g. a turning shaft or a boring bar.

[0021] Preferably, the control device is arranged to impose the dampingforce out of phase by 60°-120° alternatively 240°-300° in relation tothe oscillatory motion. Suitably, the control device is arranged toimpose the damping force out of phase by 70°-110° alternatively250°-290° in relation to the oscillatory motion. Preferably, the controldevice is arranged to impose the damping force out of phase by 80°-100°alternatively 260°-280° in relation to the oscillatory motion. In thisway, a faster damping of the oscillatory motion is obtained. Bestresults are achieved when the control device is arranged to impose thedamping force out of phase by 90° alternatively 270° in relation to theoscillatory motion.

[0022] A co-directed force should be given when the same is imposed outof phase between 60° and 120° in relation to the oscillatory motion,while a counter directed force should be imposed out of phase between240° and 300° in relation to the oscillatory motion.

[0023] Preferably, the control device is arranged to give a dampingforce in the area of 50-1500 Hz.

[0024] Suitably, at least one piezoelectric element is included in thevibration-damping device. Alternatively, the vibration-damping devicemay be a hydraulic or pneumatic cylinder or an electromagnetic device.

[0025] Preferably, said mechanical structure comprises a tool for chipremoving machining.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the following, preferred embodiments of the invention will bedescribed in detail, reference being made to the accompanying drawings,where:

[0027]FIG. 1 is a schematic side view of a long narrow body in the formof a tool shaft during bending, deformation at oscillation (1^(st)resonance frequency).

[0028]FIG. 2 is a graph showing the bending torque in the body.

[0029]FIG. 3 a side view of a cut end portion of the body in connectionwith a fastening end so as to illustrate the stress in the body duringbending deformation proportional to elongation.

[0030]FIG. 4 is a transparent perspective view of a tool shaft.

[0031]FIG. 5 is a perspective view of a bar extender for milling toolsformed with a circular cross-section.

[0032] FIGS. 6-8 are perspective views of respective tool shafts havinga square cross-section and in different alternative embodiments, whereinFIG. 6 shows a piezo element mounted in a countersink of a tool draft;FIG. 7 shows the countersink of FIG. 6 covered by a lid; FIG. 8 shows apiezo element mounted on the outside of the shaft.

[0033]FIG. 9 is a perspective view of a tool for active vibrationdamping mounted in a carrier.

[0034]FIG. 10 is an analogous perspective view of an alternativeembodiment for passive vibration damping.

[0035]FIG. 11 shows schematically the damping of an oscillatory motionby means of a counter force in phase with the oscillation.

[0036] FIGS. 12-13 show schematically the damping of an oscillatorymotion according to the invention, wherein FIG. 12 shows the applicationof a constant-amplitude damping force, whereas FIG. 13 discloses theapplication of a damping force having a gradually diminishing amplitude.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

[0037] In FIG. 1, a long narrow body in the form of a bar or a shaft 1of a tool is illustrated, which is intended to carry one or more(cutting) inserts of the tool during turning or milling. The body 1 hasa fastening end 2 and a free, external end 3. The body has an externalsurface 4, which may be cylindrical or comprise a plurality of planesurfaces if the body has a polygonal, e.g. rectangular cross-sectionshape. The body 1 may have an arbitrary cross-section shape, however,most commonly circular or rectangular. In FIG. 1, numeral 5 designates apart in which the body 1 is fastened, the body extending cantilever-likefrom the fastening part. In FIG. 1, the body 1 is shown in a state whenthe same has been deformed in a first self-bending state or “own-mode.”

[0038] Furthermore, in FIG. 2 a graph is shown which illustrates how thebending torque M_(b) in this case varies along the body. As is seen inthe graph, a maximum bending moment arises, and thus a maximum axialelongation, at or near the fastening end 2. The same is valid for alllower modes, which are normally energy-wise dominant during bendingvibrations of tools for chip removing machining.

[0039] In FIG. 3, a portion of the body 1 deformed by bending in FIG. 1is shown in the area of the fastening end. In this connection, how theelongation at bending deformation varies in the cross-direction of thebody (the elongation is strongly exaggerated for illustrative reasons)is illustrated. As is seen in the figure, the maximum elongations areobtained at the envelope surface or external surface 4 of the body.

[0040] In FIG. 4, a fundamental design of a bar or a shaft 1 isschematically shown in which two plate-formed, rectangular piezoelements 8 are fastened on opposite, longitudinal plane surfaces 4 ofthe shaft of rectangular cross-section. The piezo elements 8 are placedin the area near the fastening end 2 of the shaft. At the external end 3thereof, the shaft has a machining element in the form of a cuttinginsert 9. Thus, the piezo elements 8 are positioned in an area where themaximum axial elongation occurs during bending deformation. Althoughthis location is preferred, also other locations are feasible.Furthermore, the piezo elements 8 are oriented with the major facesthereof essentially parallel to the plane surfaces 4 of the bar or shaft1 and with the major axes essentially parallel to the longitudinal axisof the shaft or bar 1, at which the piezo elements 8 at bendingvibration will be deformed while retaining the rectangular shape.

[0041] In FIG. 5, an embodiment is shown in which the body 1 consists ofa cylindrical bar extender intended for a milling tool. In this case, achip forming machining element 9 in the form of a cutting edge is formedadjacent to a chip pocket 10 at the free end 3 of the bar extender. Apiezo element 8 is attached on the envelope surface 4 of the barextender in an area near the fastening end 2. The major axis of thepiezo element is parallel to the length extension of the bar extender.Consequently, with this orientation, the piezo element 9 acts also heremost efficiently for the damping of bending vibration.

[0042] For simultaneous damping of bending and torsional vibrations, theshaft of the tool is advantageously formed with a plurality of piezoelements, some of which being oriented with the long sides thereofessentially parallel to the length extension of the shaft, while othersare oriented at approximately a 45° angle. Alternatively, one or morepiezo elements are provided having different respective orientations.

[0043] Piezo elements are usually fragile, particularly such of theceramic type. Therefore, in demanding environments, the same should havesome form of protection in order to achieve art acceptable service life.

[0044] In FIGS. 6-8, tool shafts having a rectangular cross-section areshown, the piezo element 8 being attached and protected in alternativeways. In all cases, the piezo elements are placed in an area near thefastening part 5 (which part may consist of a conventional clamping unitin which the tool is detachably mounted).

[0045] In FIG. 6, the piezo element 8 is mounted in a countersink 11 andadvantageously covered by a protection layer, for instance of the epoxytype.

[0046] In FIG. 7, the piezo clement is assumed to be mounted in thecountersink 11 and covered by a stiff lid 12.

[0047] In FIG. 8, the piezo element 8 is mounted to, e.g. glued on, theoutside of the shaft. These alternatives should only be seen asexamples, those of which shown in FIGS. 6 and 7 being preferred. Itshould be understood that the same type of protection for the piezoelements is independent of the cross-section shape of the tool shaft.

[0048] The piezo elements co-operate with means for electric control orguiding of the same. In FIGS. 9 and 10, examples are shown of how thetool 1 has been formed with such control means. In these cases, the toolis mounted in a carrier 13. In FIG. 10, a control means for damping isshown in the form of a control means 14 formed near the fastening end 2and an electric connection 15, via which one or more piezo elements 8are connected to the control module 14 for separate or common control ofrespective piezo elements. This module 14 comprises at least electricresistive components. Preferably, the control module 14 also comprisesone or more shunts, at which selected frequencies may be dampedparticularly efficiently.

[0049]FIG. 9 illustrates a control means for active damping in the formof a free-standing logical control circuit 16, e.g. a programmablemicroprocessor, for separate or common control of (via the schematicallyillustrated electric connection 15) voltages applied over the piezoelements 8. In practice, the connection 15 may in this case comprisecollector shoes or the like.

[0050] Also, if the piezo elements 8 in the embodiment exemplified inFIG. 10 for active damping simultaneously act as both operating devicesand sensors, it is feasible to realise the same two functions byseparate operating devices and sensors, wherein the sensors do not needto consist of piezo elements. Although the depicted locations of thecontrol module 14 and logic control circuit 16, respectively, arepreferred, other locations are feasible. For instance, it is feasible,as in the case of the logic control circuit 16, to arrange the controlmodule 14 freestanding from the tool. The advantage of placing thecontrol module 14 in the vicinity of the fastening end is that themodule becomes simple to connect to the piezo elements, whereas theadvantage of arranging it at a freestanding position is that it becomeseasier to protect the module against harmful mechanical effects.

[0051] Through the usage of piezo elements as vibration dampers, arobust tool for chip removing machining is obtained having a possibilityof active damping of bending vibrations over a wide range offrequencies. Furthermore, a tool is provided which on one hand has alonger service life for the tool in itself as well as the cutting ormachining elements thereof, and on the other hand provides increasedquality of the surface on the machined workpiece. In addition, animproved working environment is attained by the reduction of highfrequency noise compared with previously known tools.

[0052]FIG. 11 shows schematically how damping of an undesiredoscillatory motion in a mechanical structure generally comes aboutaccording to afore-mentioned U.S. Ser. No. 09/913,271. If a dampingforce 20 is counter-directed to, and in phase with, the oscillatorymotion 22, the motion is quickly dampened. However, this requires a verylarge accuracy as regards phase correctness. If a phase error arisesbetween the damping force and the oscillatory motion, thecounter-directed dampening force will be partly co-directed with theoscillatory motion, which may lead to the oscillatory motion not beingquenched to the desired degree.

[0053]FIG. 12 shows schematically the damping of the oscillatory motionof the mechanical structure according to the invention. The sensordetects the oscillatory motion 22. The signal is transferred to thecontrol device, which processes the signal and determines theoscillatory motion's phase by defining positive and negative,respectively, zero crossing, The control device also calculates theamplitude and frequency of the oscillation.

[0054] The control device then sends out a control signal to thevibration damper (actuator), which generates a force 20′counter-directed relative to the tool's velocity 24. The phase isdisplaced a quarter of, alternatively three-quarters of, a wavelength inrelation to the oscillatory motion.

[0055] Since the damping force is applied in counter-direction to thevelocity of the oscillatory motion, the damping force will tend todeform the tool in that counter direction. Accordingly, regenerativevibration can be damped without a risk that the damping force willamplify the vibration, e.g., the result of an unbalanced relationshiptherebetween. The invention thus eliminates a need for a high degree ofaccuracy between the phases of the damping force frequency and theregenerative vibration frequency.

[0056] In FIG. 13, it is shown how a counter force 20″ directed in theopposite direction to that of the tool's oscillatory motion 22 can beimposed at gradually decreasing oscillation amplitude (in lieu ofconstant amplitude) to avoid a new generation of vibrations, and isthereby easier to control.

[0057] Although the present invention has been described in connectionwith preferred embodiments thereof, it will be appreciated by thoseskilled in the art that additions, deletions, modifications, andsubstitutions not specifically described may be made without departingfrom the spirit and scope of the invention as defined in the appendedclaims.

In the claims
 1. A method for damping feed-back vibrations of a metallictool during the chip removing machining of a metal workpiece comprisingthe steps of: A. causing a sensor device to detect an oscillatory motionof the tool; B. causing a control device to identify the frequency ofthe oscillatory motion detected in step A; and C. causing a vibrationdamping device to generate a mechanical damping force havingsubstantially the same frequency as the frequency identified in step Band applied to the tool in counter-direction to a velocity of theoscillatory motion.
 2. The method according to claim 1 wherein step Cincludes applying the damping force to the tool out of phase by 60°-120°relative to the oscillatory motion.
 3. The method according to claim 1wherein step C includes applying the damping force to the tool out ofphase by 240°-300° relative to the oscillatory motion.
 4. The methodaccording to claim 1 wherein step C includes applying the damping forceto the tool out of phase by 70°-110 ° relative to the oscillatorymotion.
 5. The method according to claim 1 wherein step C includesapplying the damping force to the tool out of phase by 250°-290°relative to the oscillatory motion.
 6. The method according to claim 1wherein step C includes applying the damping force to the tool out ofphase by 80°-100° relative to the oscillatory motion.
 7. The methodaccording to claim 1 wherein step C includes applying the damping forceto the tool out of phase by 260°-280° relative to the oscillatorymotion.
 8. The method according to claim 1 wherein step C includesapplying the damping force to the tool out of phase by 90° relative tothe oscillatory motion.
 9. The method according to claim 1 wherein stepC includes applying the damping force to the tool out of phase by 270°relative to the oscillatory motion.
 10. the method according to claim 1wherein the damping force generated in step C is in the range of 50-1500Hz.
 11. The method according to claim 1 wherein step C includes causinga piezo element of the mechanical damping device to generate the dampingforce.
 12. The method according to claim 1 wherein step C includescausing a hydraulic cylinder of the mechanical damping device togenerate the damping force.
 13. The method according to claim 1 whereinstep C includes causing a pneumatic cylinder of the mechanical dampingdevice to generate the damping force.
 14. The method according to claim1 wherein step C includes causing an electromagnetic device of themechanical damping device to generate the damping force.
 15. The methodaccording to claim 1 wherein step B further includes causing the controldevice to identify an amplitude of the oscillatory motion, and whereinthe mechanical damping force generated by the vibration damping deviceis of gradually decreasing amplitude.
 16. A vibration damping system fordamping feed-back vibrations of a mechanical tool during thechip-removing machining of metal workpieces, the apparatus comprising: asensor device for detecting an oscillatory motion of the tool during amachining operation; a control device operably connected to the sensordevice for identifying a frequency of the oscillatory motion sensed bythe sensor device; and a vibration damping device for generating amechanical damping force having substantially the same frequency as theoscillatory motion and a counter-direction relative to the direction ofthe velocity of the oscillatory motion.
 17. The vibration systemaccording to claim 16 wherein the control device identifies an amplitudeof the oscillatory motion, and the vibration damping device generates adamping force of gradually decreasing amplitude.
 18. A metal cuttingapparatus for the chip removing machining of metal workpieces, theapparatus comprising: a metal-cutting tool; and a vibration dampingsystem operably connected to the tool and comprising: a sensor devicefor detecting an oscillatory motion of the tool during a machiningoperation; a control device operably connected to the sensor device foridentifying a frequency of the oscillatory motion sensed by the sensordevice, and a vibration damping device for generating a mechanicaldamping force having substantially the same frequency as the oscillatorymotion and applied to the tool shaft in counter-direction to a velocityof the oscillatory motion.
 19. The apparatus according to claim 18wherein the damping force is applied to the tool out of phase by60°-120° relative to the oscillatory motion.
 20. The apparatus accordingto claim 18 wherein the damping force is applied to the tool out ofphase by 240°-300° relative to the oscillatory motion.
 21. The apparatusaccording to claim 18 wherein the damping force is applied to the toolout of phase by 70°-110 relative to the oscillatory motion.
 22. Theapparatus according to claim 18 wherein the damping force is applied tothe tool out of phase by 250°-290° relative to the oscillatory motion.23. The apparatus according to claim 18 wherein the damping force isapplied to the tool out of phase by 80°-100° relative to the oscillatorymotion.
 24. The apparatus according to claim 18 wherein the dampingforce is applied to the tool out of phase by 260°-280° relative to theoscillatory motion.
 25. The apparatus according to claim 18 wherein thedamping force is applied to the tool out of phase by 90° relative to theoscillatory motion.
 26. The apparatus according to claim 18 wherein thedamping force is applied to the tool out of phase by 270 °relative tothe oscillatory motion.
 27. The apparatus according to claim 18 whereinthe damping force is in the range of 50-1500 Hz.
 28. The apparatusaccording to claim 18 wherein a piezo element of the mechanical dampingdevice generated the damping force.
 29. The apparatus according to claim18 wherein a hydraulic cylinder of the mechanical damping devicegenerates the damping force.
 30. The apparatus according to claim 18wherein a pneumatic cylinder of the mechanical damping device generatesthe damping force.
 31. The apparatus according to claim 18 wherein anelectromagnetic device of the mechanical damping device generates thedamping force.
 32. The apparatus according to claim 18 wherein thecontrol device identifies an amplitude of the oscillatory motion, andthe vibration damping device generates a mechanical damping force ofgradually decreasing amplitude.